Process of doping silver image in chalcogenide layer

ABSTRACT

A layer of a chalcogenide glass resist material, on a substrate on which a microlithographic pattern is to be formed, has a deposit of silver halide on its outer surface. By actinic irradiation a latent silver image replicating the desired pattern is formed in the silver halide deposit. This image is developed to a metallic silver, which is used to photodope the resist material for subsequent etching to produce the microlithographic pattern on the substrate. Positive and negative patterns are possible from the same starting laminate. One form of a microlithographic pattern is a mask for producing electronic circuits.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 57,183 filed July13, 1979, now abandoned.

BACKGROUND OF THE INVENTION

In the fabrication of microlithographic patterns, for use, as examples,in photoetching processes (e.g., processing semiconductor integratedcircuits and in production of electronic circuits) there is a continuingneed to improve photoresist materials in order to create and toreplicate high-density information patterns on semiconductor bodies andother substrates. Properties of increasingly high resolution, fast andfaithful response to control energy whether in the form of a plasma, anelectron beam, heat energy, visible light, or other, are all desired.

Chalcogenide glass films, such as Se-Ge glasses, As₂ S₃ glasses or otherbinary glassy compounds of oxygen, sulfur, selenium and tellurium, havebeen found to exhibit a selective etching effect due to photoexposuresuch that they are suitable for use as photoresist materials. Beingglass, i.e., amorphous in structure, they exhibit also high resolutioncapabilities suitable for microlithographic applications (Nagai, et al,"New application of Se-Ge glasses to silicon micro-fabricationtechnology," Applied Physics Letters, Vol. 28, No. 3, Feb. 1, 1976,pages 145-147).

It is known that a thin (less than 100 Angstrom units) layer of metallicsilver on a chalcogenide glass acts to "photodope" the glass, as thesilver can be driven into the glass to improve the photoresistproperties. When silver is in the chalcogenide glass in patterns, thereis differential resistance to etching compositions. (Yoshikawa, et al,"A novel inorganic photoresist utilizing Ag photodoping in Se-Ge glassfilms," Applied Physics Letters, Vol. 29, No. 10, Nov. 15, 1976, pages677-679). The same effects of photodoping were later observed inelectron-beam irradiation (Yoshikawa, et al. "A new inorganic electronresist of high contrast, "Applied Physics Letters, Vol. 31, No. 3, Aug.1, 1977, pages 161-163). Electron irradiation on a double-layered filmof Ag and Se-Ge glass, like irradiation with light, gives rise todriving silver into the underlying Se-Ge layer.

When silver is forced into a chalcogenide glass in patterns for etching,the silver-doped chalcogenide becomes insoluble in alkaline solutions.It has been suggested that silver halide can be used in place of silverto supply the silver for photodoping through dissociation into metallicsilver and halogen gases upon irradiation, and a 20 nm thick layer ofAgCl deposited from vapor on a glass film of As₂ S₃ and 300 nm thick hasbeen reported to be useful for photodoping, but its sensitivity, orphotographic speed is still slower than desired (Kolwicz and Chang"Silver Halide-Chalcogenide Glass Inorganic Resists for X-RayLithography"--unpublished).

When Se-Ge glass is etched after conventional photoexposure the resultis a photographically positive photoresist, or a resist in which theexposed areas of the glass are etched away. At least one report statesthat the opposite is true of an As₂ S₃ glass where conventionalphotoexposure and etching yield a negative resist (Kolwicz and Chang,supra). However, when the glass is "photodoped" and silver is driveninto the glass by a pattern of actinic radiation, both the Se-Ge glassand the As₂ S₃ glass form a negative resist. (Yoshikawa, et. al.,Applied Physics Letters, Vol. 29, No. 10, pages 677-679).

GENERAL DESCRIPTION OF THE INVENTION

In the present invention a composite resist made of silver halide onchalcogenide glass is used photographically to provide a novelmicrolithographic process which is faster than any heretofore available,together with several novel intermediate products. The composite resistcan be processed to a photographic positive or a photographic negativewhich replicates the desired pattern. Silver is provided by a process ofexposure of the silver halide to actinic radiation followed byphotographic processing to provide silver in a pattern. This silverpattern can be used to photodope the underlying glass layer with theapplication of heat, as by irradiation with an infrared flood light orby another method of causing the silver to penetrate into the glass.Production of the latent silver image in the silver halide requires muchless energy than is required to bring about direct dissociation ofsilver halide into metallic silver and a gas and the present inventionis thus "faster" in the photo-responsive sense, thus generating animportant manufacturing advantage.

The silver halide deposit is not necessarily and not usually in the formof a continuous layer. This deposit generally originates in the form ofseparate droplets or islands of silver halide, which may be in the 0.1micron range, and only as this deposit becomes thicker do these islandstouch each other and then overlap to become an essentially continuouscovering or layer. This result is particularly true with chalcogenideglass substrates such as arsenic trisulfide which apparently is notwetted by the depositing silver halide. When this deposit is in the formof multitudinous islands of silver halide, and has an effectivethickness well under 0.3 micron, it operates in a fully satisfactorymanner and, in fact, in some ways produces better results. In contrast,prior patents relating to the making of silver halide photographiclayers by deposition from the vapor phase have proposed continuouslayers, and such continuity has previously been considered important.LuValle et al., U.S. Pat. No. 3,219,448, issued Nov. 23, 1965; Col. 9,lines 39-45). In the present invention, the silver halide deposit can bemuch thinner than prior art photographic layers, such as in the range of100 Angstrom units or less. It is preferably described as a depositdistributed over the outer surface of the chalcogenide resist material,so as to include within its scope either a continuous layer or adistribution of aggregates of microcrystals which do not necessarilytouch each other. The invention makes possible the design of a compositeresist of silver halide on chalcogenide glass, which will permitdevelopment of a photographically latent silver image into silver foruse in microlithography and at the same time provide sufficient silverfor the photodoping step that follows, preferably without providingsubstantially more silver than is necessary for adequate photodoping ofthe immediately-underlying glass.

The invention is described in detail with reference to the accompanyingdrawings, which illustrate the prior art and some embodiments of theinvention as it is best understood at the present time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 inclusive illustrate the prior art.

FIG. 1 shows a chalcogenide resist on a substrate, being irradiated asthrough a mask;

FIG. 2 illustrates the product formed from FIG. 1 after irradiation andtreatment, to provide a pattern of resist replicating the pattern in themasked surface of the substrate;

FIG. 3 shows a second stage in the prior art wherein a silver overcoatis provided over the resist prior to irradiation or subsequenttreatment;

FIG. 4 illustrates the irradiation of the article in FIG. 3;

FIG. 5 illustrates the pattern of irradiation fixed on a surface of thesubstrate after treatment of the product shown in FIG. 4;

FIGS. 6 to 18, inclusive, illustrate embodiments of the presentinvention;

FIG. 6 illustrates the starting article comprised of a substrate havinga chalcogenide resist layer on it and on an outer layer of the resist adeposit of silver halide;

FIG. 7 illustrates exposure of the silver halide deposit to actinicradiation, as through a mask, to provide a latent image of the patternin the mask, in the silver halide deposit;

FIG. 8 illustrates a silver replication of the mask formed by developingthe latent image of FIG. 7 into a metallic silver pattern which is anegative replication of the pattern of the mask;

FIG. 9 illustrates the substrate with chalcogenide resist on it bearingthe metallic silver negative replication of the mask after fixing andremoving the unexposed and undeveloped silver halide;

FIG. 10 illustrates the metallic silver pattern "doped" into thechalcogenide resist;

FIG. 11 illustrates the negative positive pattern of the mask replicatedin the resist material after etching and treating the resist to removethat portion unprotected by the metallic silver doping mask;

FIGS. 12 and 13 are illustrations of the general nature of discontinuousthin silver halide deposits;

FIG. 14 illustrates a second embodiment of the invention wherein apositive pattern is formed in the resist; in FIG. 14 the silverreplication of a mask as existing in the intermediate article shown inFIG. 8 has been removed, and the unexposed silver halide has beenallowed to remain on the resist;

FIG. 15 illustrates a latent image formed in the previously unexposedsilver halide;

FIG. 16 illustrates a metallic silver image formed from the latent imageof FIG. 15;

FIG. 17 illustrates the negative pattern reproduced in the resist afterphotodoping, etching and treatment to remove that portion of the resistnot covered by the doped-in silver; and,

FIG. 18 illustrates a starting article for practicing the inventionwhich includes a protective overcoat covering the silver halide deposit.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1-5, illustrating the prior art, a substrate 10,which for the purposes of the present disclosure is a semiconductormaterial on which it is desired to form a pattern with microlithographictechniques, and which is coated with a chalcogenide resist material 12,is irradiated with suitable energy in a limited region defined betweentwo downwardly arrows 14, which may be accomplished by the use of asharply-focused electron beam (not shown) or through a mask 16 as shownhaving an appropriate aperture to define the region between the twoarrows 14. As is known in the art, appropriate irradiation will alterthe character of a chalcogenide resist material quantitatively so thatwhen treated with an etchant the resist material will be more resistantto the etchant in the region where irradiated than in other regions withthe result that, as is illustrated in FIG. 2, subsequent removal of theresist material with an appropriate etchant will differentially removethe resist material and a thinner portion of the resist material 12'will be left marking the pattern of the irradiation on a surface of thesubstrate 10.

FIGS. 3, 4 and 5 represent the subsequent development in the artaccording to which a silver overcoat 18 is provided on the resistmaterial 12 prior to irradiation, and the irradiation has the effect of"driving" some silver into the underlying resist layer so as to dope theunderlying layer with silver particles, as is illustrated at 20. This isknown as "photodoping" the chalcogenide resist. Resistance to subsequentetching is increased in this manner so that, as is shown in FIG. 5,subsequent treatment with an etchant to remove the portion of thechalcogenide resist material which was not exposed to irradiation leavesthe exposed portion 12' under the silver dopant 20 substantially asthick as the original chalcogenide resist layer 12. One result is thatfiner resolution can be achieved and that processing is less susceptibleto error.

The present invention uses as its starting article the combination shownin FIG. 6 wherein a deposit of silver halide 22 is provided on thechalcogenide resist layer 12. When the end product is to be used as amask, a presently preferred substrate is quartz. As is pointed out abovein the review of the prior art, it has heretofore been proposed toemploy a layer of silver halide in place of the metallic silver overcoatthat is illustrated in FIG. 3, and to use that layer to provide metallicsilver in response to irradiation. In the present invention, the silverhalide deposit 22 is irradiated with actinic radiation, either with anelectron beam (not shown) or through a suitable apertured mask 16, toproduce in the silver halide deposit a latent silver image 24,replicating the pattern desired to be imposed on the substrate 10. Thisis illustrated in FIG. 7. FIG. 8 shows the developed metallic silverimage 26 on the outer surface of the resist material 12. By photographictechniques the image can be fixed, and the unexposed silver halideremoved, to provide a metallic silver replication of a desired patternon the resist material, as is illustrated in FIG. 9. At this stage heat,such as infrared radiation according to the prior art, can be employedto photodope the resist material 12 in regions 28 immediately underlyingthe metallic silver pattern 26, as is illustrated in FIG. 10.Irradiation is presently preferred over simple heating, partly becauseirradiating may tend to drive the silver directly into the chalcogeniderather than merely letting the silver diffuse into the glass, thusproducing cleaner, sharper lines. FIG. 11 shows the end product ofetching away the unprotected resist material, this step being inaccordance with the prior art, leaving islands of resist material 12'covered with doped regions 28 corresponding to regions 20 as areillustrated in FIG. 5.

"Actinic radiation" as used in this specification is any radiation whichcan form a latent image in the silver halide to produce a developableimage; this includes electron beams, ion beams, x-rays, light, and thelike.

The step of fixing the developed image 26 may be omitted, and the silverhalide deposit 22 with the metallic silver image 26 in it may beirradiated with infrared radiation to photodope the substrate 12, givingthe product shown in FIG. 10. The silver halide is transparent toinfrared radiation, and remains on the surface 32 (not shown in FIG.10). Thereafter, the silver halide can be removed in the same step whichetches away the resist material that is not protected by the doped-insilver. Suitable agents for this simultaneous fixing and etching stepare ammonium hydroxide (for AgCl and AgBr), or an alkaline photographicfixing agent.

The silver halide layer 22 is not necessarily a continuous layer. Theamount of silver required to provide effective doping as at 20 or 28 inFIGS. 5 and 11, respectively, is believed to be small. The photographicprocess employing a silver halide photographic material is not onlyfaster than processes heretofore used in making microlithographicpatterns on semiconductors and the like; it is also capable of producinglarge amounts of silver out of very small amounts of silver halide byphotographic techniques. Accordingly, the silver halide deposit 22 canbe the result of distributing silver halide over the outer surface ofthe resist material 12 to a thickness ranging from 100 angstrom units toa thickness that is known to be useful for photographic purposes(exemplified by approximately 0.3 micrometer). For the present inventiona layer substantially thinner than 0.3 micrometer is preferred generallyabout 0.1 micrometer and as a preferred thickness between about 0.05 andabout 0.2 micrometer. A very thin layer may produce cleaner, finer linesin the final product. At the lower end of the thickness range, there canbe a "layer" which is not continuous and in which there are particlesthat are separated from all neighboring particles. Such a distributionis illustrated in FIG. 12 in which aggregates 30 of silver halide areshown distributed over the surface 32 of the underlying resist material12. The aggregates are in effect micro-sized clumps of silver halidewhich do not necessarily touch each other. A more compacted distributionis illustrated in FIG. 13 in which small aggregates 34 of silver halideare shown on the surface 32 but more closely packed together and havinga large proportion of them touching each other. This does not excludethe possibility of a continuous layer of silver halide material at theupper end of the thickness range.

FIGS. 14 to 17, inclusive, illustrate a second embodiment of theinvention, in which a positive replication of the desired pattern isformed on the substrate 10. Beginning with the process as it isillustrated in FIGS. 7 and 8, this embodiment removes from theintermediate article shown in FIG. 8 the metallic silver replication 26,leaving the undeveloped silver halide 22 in place as is shown in FIG.14. Metallic silver can be removed by use of a light application ofnitric acid or other silver bleach agent, or a silver-dissolving agent,several of which are known in photographic art. Then as is illustratedin FIG. 14, the silver halide 22 that remains is irradiated with a floodlight (visible light is adequate), to produce a latent image 36,consisting essentially of silver atoms in a matrix of silver halide, onthe surface 32 of the resist material 12, as is illustrated in FIG. 15.Then, by the usual photographic process of developing to a silver image38 and fixing, a metallic silver replication of a positive image isformed, as is shown in FIG. 16. This metallic image can then be used todope the underlying resist material 12 forming doped regions 40.Thereafter, the uncovered regions of the resist material 12 are etchedaway to provide islands 12" delineating the photolithographic patternthat is desired on the substrate material 10. The benefit of the secondembodiment of the invention is that it provides for the first time theopportunity to obtain a positive or a negative mask on a semiconductorchip, using the same starting chalcogenide resist material.

Silver halide layers, especially very thin silver halide layers, arefragile and can be damaged in operations such as dipping into nitricacid or into sodium hydroxide etchants. For that reason, it may bedesired to use the silver halide layer 22 with a material such as agelatin to give it physical protection while permitting chemical andphysical operations on it. Such a protective overcoat 42 is illustratedin FIG. 18.

The following example is presented to illustrate the production of amicrolithographic product such as a mask for the production of amultiplicity of micro semiconductor chips.

EXAMPLE 1

A quartz plate, or other substrate, is placed in a vacuum system whichis then evacuated to a pressure between 5×10⁻⁶ and 1×10⁻⁵ Torr. Arsenictrisulfide is evaporated from a molybdenum boat at a rate of 10 to 20 Aper second as measured on a quartz crystal thickness monitor.Evaporation is continued to obtain a layer of arsenic trisulfide about0.3 micrometer thick. The thickness of the arsenic trisulfide layer isnot critical, and a uniform layer between 0.1 and 0.5 micrometer issought.

A layer of silver bromide is evaporated over the arsenic trisulfide. Ata pressure of approximately 1×10⁻⁵ Torr, silver bromide is evaporatedfrom a tungsten boat held at a temperature of 615° C., to a thickness ofabout 100 A. The product is a three layer member comprising a quartzplate, an arsenic trisulfide layer and a top layer of silver bromide.

This three layer product is exposed to suitable actinic radiation, forexample to direct writing of a pattern with an electron beam. In oneembodiment the beam pattern represents a semiconductor electroniccircuit. The exposed layer is developed in a solution containing:

hydroquinone: 2.5 g/1000 ml.

p-methylaminophenol sulfate: 0.67

sodium sulfite: 26

sodium carbonate: 26

gelatin: 1.67

for a period of 30 seconds. At this point there are several options,depending on the nature of the product desired and depending on desiredprocedures. In this example, the developed and unfixed image is exposedto a 250 W. infrared lamp for 30 minutes (filtered to be sure that allvisible light is removed), to drive the silver image into the arsenictrisulfide layer to form a photo-doped arsenic pattern.

Alternatively, the developed silver image can be removed by placing theplate in 4 molar nitric acid for 30 seconds. The remaining silverbromide is then fogged by exposure to light or other actinic radiation,and is developed again in a like developer solution to form an image ofopposite sense. The resulting silver image is then driven into thearsenic trisulfide layer by exposure to infrared light. As anotheralternative, the silver bromide can be removed from the originaldeveloped image, and the plate either flooded with infrared light orstored for later treatment.

After photo-doping, the arsenic trisulfide layer is etched byconventional methods to form an etched layer of arsenic trisulfide on aquartz substrate, suitable for use as a mask for the production ofsemiconductor electronic circuits. The optical density of the patternreaches a value of 1.0 for radiation at about 420 micrometer and a valueof 2.0 at about 393 micrometer. Mercury lines at 365 micrometer or 405micrometer will be sufficiently absorbed by the pattern to preventexposure in the masked areas.

The preceding Example is illustrative of procedures for placing anarsenic trisulfide glass, a germanium selenium glass or otherchalcogenide glass on a suitable substrate, for depositing thereon asilver halide layer, and for exposing and developing such a layer toform a developed photographic silver image on the glass. Thereafter, thesilver image is caused to move into the glass, forming a photo-dopedchalcogenide glass layer on the substrate. As is known, the photo-dopedchalcogenide glass can then be etched to form a microlithographicresist.

We claim:
 1. In a process employing a chalcogenide glass to delineate apattern, the steps of:(a) distributing a silver halide deposit over theouter surface of the chalcogenide glass; (b) exposing portions of saidsilver halide deposit to a pattern of actinic radiation on said silverhalide deposit to produce a latent image corresponding to a desiredpattern in said silver halide deposit; (c) photographically processingsaid latent image to form a metallic silver replication of said pattern;and (d) causing said metallic silver replication to penetrate into thesurface of said chalcogenide glass to dope with silver theimmediately-underlying surface of said chalcogenide glass.
 2. Theprocess of claim 1, wherein the silver-doped chalcogenide glass is thenselectively etched.
 3. A process according to claim 1 including the stepof removing undeveloped silver halide from said deposit after developingsaid latent image and before doping said resist material.
 4. A processaccording to claim 1 wherein said silver halide deposit is distributedin individual islands on said chalcogenide glass to an effectivethickness ranging from about 100 Angstrom units to about 0.3 micrometer.5. A process according to claim 1 including the additional step ofsimultaneously removing the unexposed silver halide of said deposit andportions of said resist material immediately underlying said unexposedsilver halide.
 6. A process according to claim 1 employing infra-redradiation to dope said resist material.
 7. A process according to claim1 wherein said chalcogenide material is selected from Se-Ge and As₂ S₃chalcogenides.
 8. In a process employing a chalcogenide glass materialto delineate a pattern, the steps of:(a) distributing a silver halidedeposit over the outer surface of the chalcogenide glass; (b) exposingportions of said silver halide deposit to actinic radiation whichreplicates said pattern on said silver halide deposit to produce alatent image of said pattern in said silver halide deposit; (c)developing said latent image to provide a metallic silver replication ofsaid pattern in said silver halide deposit; (d) removing said metallicsilver from said deposit; (e) producing a latent silver background imagein the previously unexposed silver halide of said deposit; (f)developing said latent silver background image to provide a metallicsilver background image; and (g) silver-doping said chalcogenide glassin said metallic silver background image areas.